Power consumption, "P", of a data processing system is commonly calculated in terms of a power supply voltage such as P=CV.sup.2 f, where "P" is power in watts, "C" is capacitance in farads, "V" is a power supply voltage in volts, and "f" is frequency of operation in hertz. For example, if the data processing system operated with a nominal five volt power supply and internally switched 10 pico-farads at a rate of one mega-hertz, the power consumption would be: P=(10.sup.-12)(5.sup.2)(10.sup.6)=25.times.10.sup.-6 watts, or 25 micro-watts. Since the power is proportional to the square of the voltage, a substantial reduction in power is achieved by simply reducing the power supply voltage.
A known method of reducing power within a data processing system that operates at a standard power supply voltage of five volts is through the use of a precharge circuit which couples a predetermined portion of the power supply voltage to a precharge circuit node. For example, when an N-channel MOS transistor has a control electrode voltage of five volts and a drain electrode voltage of five volts, a source electrode thereof will be at a potential of five volts minus an N-channel threshold voltage (V.sub.tn). For example, if the V.sub.tn is one volt, the source electrode voltage would be four volts. Using the above power formula and parameters with a reduced power supply voltage of four volts, the power is calculated to be 16 micro-watts. Therefore, a 20% reduction in internal operating power supply voltage yields a 36% decrease in power consumption. An additional advantage to reducing the voltage on precharge circuit nodes within the data processing system is an increase in potential operating speed. A reason why the potential operating speed of the data processing system is increased is that the precharge voltage level is typically closer to a switch point of logic circuits connected to the precharge circuit node, where a switch point is defined as a necessary input voltage of a circuit to cause an output of the circuit to switch. If we assume that a typical switch point for circuits within a data processing system connected to a precharge circuit node is at one-half the power supply voltage of five volts, and that V.sub.tn is one volt, the precharge voltage level is therefore 1.5 volts above the switch point of logic circuits whose inputs are connected to the precharge circuit node. In contrast, if the precharge voltage level on the precharge circuit node were at the power supply voltage of five volts, the voltage difference between the precharge voltage level and the switch point would be 2.5 volts. It is known that when using a transistor to precharge a node that provides a voltage level close to the switch point of logic circuits connected to the precharge circuit node, the data processing system has the potential of operating faster. Further, the voltage difference between the precharge voltage level and the switch point of the circuit is termed "noise margin." For safest possible circuit operating conditions within the data processing system it is desired to have a maximum possible noise margin. However, in the design of data processing systems it is common to balance noise margin safety with increases in circuit speed performance. A known problem with utilizing transistors that have a V.sub.tn of one volt for precharging nodes is that if a further reduction in power consumption is desired by reducing the operating power supply voltage of the data processing system, the noise margin is substantially reduced. For example, if the operating power supply voltage were decreased to three volts, and the switch point of the circuits remained at one-half the power supply voltage, i.e. one and one-half volts, the difference between the precharge voltage, V.sub.dd -V.sub.tn, and the switch point voltage would only be one-half volt. It is important to note that the threshold voltage of MOS transistors are determined during a manufacturing process, and variances in threshold voltages are common during processing. Therefore, a small noise margin value, such as the one-half volt, is generally not considered adequate.
It is therefore desired to have a data processing system manufactured in a known predetermined process with transistors having a V.sub.tn such that speed performance is achieved when utilizing precharge circuits, and yet will still operate with adequate noise margins with a minimum valued power supply voltage.